I have a generic VFD for a 7.5 HP 460V motor 1770 RPM with 9.8 FLA and 1.15 SF on an air handler. When the VFD is running 38-40 HZ the motor is at 9.8 FLA. Is the motor running at or near 1770 RPM? I know 1HZ = 60 RPM. The only way I can think to change this is with a pulley swap to lower the amperage to run at 60 HZ which would be near 1770 RPM.
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VFD hertz vs FLA
- Thread starter Lsshvacr
- Start date
- Replies 7
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johansen
Stainless
- Joined
- Aug 16, 2014
- Location
- silverdale wa
At 40hz he motor is around 1100rpm.
change the pulley to get the rpm up and the torque down so the motor runs more efficiently.
change the pulley to get the rpm up and the torque down so the motor runs more efficiently.
SomeoneSomewhere
Hot Rolled
- Joined
- Dec 24, 2019
This is a four-pole motor, so it's 30RPM per Hz, rather than 60. We know this because the nameplate speed is ~1800RPM at 60Hz, not ~3600RPM.
40Hz gets you ~2/3 speed, or a bit under 1200 RPM.
You'll need to screw with the pulleys.
40Hz gets you ~2/3 speed, or a bit under 1200 RPM.
You'll need to screw with the pulleys.
Just to understand this situation..... some more information is needed to give the best answer.
I assume this is a replacement motor? If so, why was it replaced? was it the same rpm? is it the same power?
If the original was 6 pole, then you need to change the pulley to get the motor to full rpm/power at 60 Hz while airflow is as-desired.
But, if not, then why the 40 rpm? Is it that the thing stalls out there?
Is the airflow as-desired at 40 Hz? That would suggest the original motor was 1200 rpm nominal/6 pole.
If it is programmed to go to 60, but stalls at 40, and the airflow is not as much as desired, then the motor is still accelerating at that point. Higher current is normal during acceleration, if it just trips there.
Of course if the original was 1200 rpm, then either operate this one at 40 Hz, or change the pulley. I'd change the pulley, so the motor has best cooling.
if the original motor was same power and rpm, what do you have the basic VFD settings at? (Motor voltage, motor rated Hz/speed, motor rated current)
Air movers have a pretty steep power needed vs rpm curve, so they can stall out a motor at a particular rpm. Fan motors are designed for higher slip so that they can operate correctly with different temperatures and air densities, just for that reason.
I assume this is a replacement motor? If so, why was it replaced? was it the same rpm? is it the same power?
If the original was 6 pole, then you need to change the pulley to get the motor to full rpm/power at 60 Hz while airflow is as-desired.
But, if not, then why the 40 rpm? Is it that the thing stalls out there?
Is the airflow as-desired at 40 Hz? That would suggest the original motor was 1200 rpm nominal/6 pole.
If it is programmed to go to 60, but stalls at 40, and the airflow is not as much as desired, then the motor is still accelerating at that point. Higher current is normal during acceleration, if it just trips there.
Of course if the original was 1200 rpm, then either operate this one at 40 Hz, or change the pulley. I'd change the pulley, so the motor has best cooling.
if the original motor was same power and rpm, what do you have the basic VFD settings at? (Motor voltage, motor rated Hz/speed, motor rated current)
Air movers have a pretty steep power needed vs rpm curve, so they can stall out a motor at a particular rpm. Fan motors are designed for higher slip so that they can operate correctly with different temperatures and air densities, just for that reason.
This is on a new air handler with correct motor for fan curve. Desired RPM is give to take 1600. Usually the VFD comes installed, matched, and programmed with the air handler. This is one is not. The VFD is setup correctly for the 7.5 HP and 460V motor. Because of supply chain issues we are having to piece together systems to keep up with construction progress. The VFD is sized correctly for the motor, but is not necessarily what it would come with standard. So I was just trying to confirm my thought that an adjustment of the pulley to lower the amperage, so that I can run the desired RPM at 60 hertz. If that makes sense.
SomeoneSomewhere
Hot Rolled
- Joined
- Dec 24, 2019
What's the designed impeller RPM, and is it hitting that? Is it operating with final duct pressure losses, or in open air?
If the system is intended to operate at 1600RPM with the current belt ratio, it seems that the motor is undersized. It's unlikely that a change in belt is going to get you enough of a torque decrease to allow operation 30% faster, impeller speed.
If you are testing in open air (with no duct attached), bear in mind that this significantly increases airflow and increases power draw/torque for a given impeller RPM.
If the system is intended to operate at 1600RPM with the current belt ratio, it seems that the motor is undersized. It's unlikely that a change in belt is going to get you enough of a torque decrease to allow operation 30% faster, impeller speed.
If you are testing in open air (with no duct attached), bear in mind that this significantly increases airflow and increases power draw/torque for a given impeller RPM.
johansen
Stainless
- Joined
- Aug 16, 2014
- Location
- silverdale wa
Blowers run on the square and or cube of the rpm depending on how much air you let flow through the blower and other details like forwards or backwards blades..
5hp at 1100 rpm is going to suggest minimum of 10hp at 1600rpm.
5hp at 1100 rpm is going to suggest minimum of 10hp at 1600rpm.
A pulley change should allow you to run the motor at it's 60 hz design speed.
The question is what the blower rpm is going to be, and what the air movement will be. You can "do as well as you can" with that motor etc, but you may have to do a bit of pulley selection to get to FLA at 60 Hz without exceeding the limits.
The "as well as you can" sounds as if it kinda has to be good enough, if you cannot get what is supposed to be installed.
The point about open flow is extremely important. If it is running open flow, the load is maximum and almost surely more than the system was designed to have as a load. Presumably you already know that, sounds like it is not the first system you have dealt with.
If the test does not duplicate the actual use conditions, you really don't know what will happen. If it is installed in the end use situation, then you can try tweaking to get as close to what is supposed to happen as possible. Presumably there are some specs on intended airflow, etc.
The question is what the blower rpm is going to be, and what the air movement will be. You can "do as well as you can" with that motor etc, but you may have to do a bit of pulley selection to get to FLA at 60 Hz without exceeding the limits.
The "as well as you can" sounds as if it kinda has to be good enough, if you cannot get what is supposed to be installed.
The point about open flow is extremely important. If it is running open flow, the load is maximum and almost surely more than the system was designed to have as a load. Presumably you already know that, sounds like it is not the first system you have dealt with.
If the test does not duplicate the actual use conditions, you really don't know what will happen. If it is installed in the end use situation, then you can try tweaking to get as close to what is supposed to happen as possible. Presumably there are some specs on intended airflow, etc.
